There is a need in science and technology for short- and long-wavelength cut-off filters. An ideal cut-off filter would have a very sharp transition between the transmission and rejection regions. It would have very small losses in the transmission region and a high attenuation or reflectance in the rejection region over an extended spectral range. At oblique angles of incidence its performance would be independent of the polarization of the incident light and it would have a reasonable angular field.
For the visible range of wavelengths there is a very complete set of short-wavelength cut-off filters based on absorption in colored glasses or organic dyes. They are relatively cheap and convenient to use. Many companies provide such filters.
Unfortunately, a corresponding set of long-wavelength cut-off filters does not exist for the visible and adjacent near infrared spectral regions. In the past, solutions to this problem have been sought that are based on the interference of light in thin films. The thin film layers are usually deposited onto a plate glass substrate and they may or may not be protected with an additional cover plate. When the light is incident at an oblique angle, a second type of solution is possible in which the coating is deposited at the interface of two cemented prisms. Interference cut-off filters can be constructed with the transition occurring at any desired wavelength, and almost any desired rejection can be achieved provided that a sufficiently large number of layers is used. The disadvantage of these devices is that, in order to achieve a rejection over a wide range of wavelengths, a number of contiguous layer stacks have to be used and consequently the total number of layers in the resulting system can be quite large. Another disadvantage is that the range of wavelengths over which the transmittance is high can be relatively narrow, unless special designs are used, in which case the number of layers required is even larger.